Composite structures have been known in the art for many years. Although composite structures can be formed in many different manners, one advantageous technique for forming composite structures is a fiber placement or automated collation process. According to conventional automated collation techniques, one or more ribbons of composite material (also known as composite tows) are laid down on a substrate. The substrate may be a tool or mandrel, but, more conventionally, is formed of one or more underlying layers of composite material which have been previously laid down and compacted. In this regard, conventional fiber placement processes utilize a heat source to assist in compaction of the plies of composite material at a localized nip point. In particular, the ribbon or tow of composite material and the underlying substrate are heated at the nip point to increase the tack of the resin of the plies while being subjected to compressive forces to ensure adhesion to the substrate. For example, the plies of composite material can be compacted by a compliant pressure roller as described by U.S. Pat. No. 5,058,497, which is incorporated herein by reference. To complete the part, additional strips of composite material can be applied in a side-by-side manner to form layers and can be subjected to localized heat and pressure during the consolidation process. Other conventional fiber placement process methods are described in U.S. Pat. No. 5,700,337, which is incorporated herein by reference.
Composite laminates that are fabricated by the fiber placement process are typically subjected to a 100% ply-by-ply visual inspection for such defects as tow gaps, overlaps and twists. Typcially, the inspection is performed manually by either an inspector or the fiber placement machine operator. The machine must be stopped and the process of laying materials halted until the inspection is complete. During the inspection, the operator verifies the dimensions of any suspect anomalies and quantifies the number of anomalies per given unit area. The anomalies are repaired as needed and laying of the next ply proceeds. However, the fabrication process has been disadvantageously slowed by the inspection process.
To overcome the disadvantages of manually inspecting a workpiece, machine inspection systems have employed video and other images that are processed by a computer to detect the existence of irregularities on an inspected object. For example, U.S. Pat. No. 4,760,444 discloses a machine visual inspection device having video inspection stations for determining the reflectance of different portions of a workpiece. A central processing unit then digitizes the reflectance values and stores the digitized values in memory. The computer also contains a standard image previously stored in memory that serves as a reference to the reflectance values. As such, the computer can compare the standard image to the digitized reflectance values to located any anomalies. However, this system provides only a single reference point when inspecting workpieces that cannot be modified by the operator.
Another inspection system is disclosed by U.S. Pat. No. 4,064,534, which discloses a television camera and logic circuitry to electronically compare the profile of an image of a workpiece against a standard image whereby the item being inspected or measured can either be rejected or accepted. More specifically, a video image of the workpiece is captured by a TV camera and converted into digital form for recording in a memory device. The recorded image is then compared against a standard image that is preloaded into memory. Based on the differences between the images, a processor determines whether the workpiece passes or fails. However, this system also requires that the standard measurements are preloaded into the computer and not controllable by the operator thereafter.
Yet another conventional inspection system employs a laser that is swept across a workpiece to identify locations on the workpiece where laser reflectivity changes. For example, a gap or other inconsistency would cause a change in the reflectivity of the surface. The reflectivity changes are then interpreted by a computer to identify defects.
However, each of these systems is susceptible to obtaining false readings due to glare or other problems caused by ambient lighting or by the laser-based scanning system. In this regard, conventional machine-based inspection systems lack a suitable lighting component that provides high contrast for defects located on the workpiece, while preventing ambient lighting and material reflectivity from hampering the identification of defects. This is further complicated during inspection of carbon materials by the appearance of black defects on a black background. In addition, conventional machine-based inspection systems do not readily permit the definition of defects or the viewing area to be altered in a controlled manner.